[time-nuts] 10 MHz Phase Coherent

[buehl]

At 06:24 PM 5/25/2007 +0200, you wrote:
>Hi Martyn,
>
>I work with other people in the timing system for CERN's particle
>accelerator complex in Geneva (Switzerland), which includes the Large
>Hadron Collider (LHC), hopefully to start operation in spring 2008. Our
>timing system consists basically of a lot of complicated high level
>software orchestrating the transfer of particles from one accelerator to
>another, and the subsequent acceleration cycles. Hardware-wise, we have
>home-made VME modules which receive the 10 MHz and PPS signals from a
>commercial GPSDO (Symmetricom XLi) and use these two signals to encode
>messages (as decided in advance by the high level software) on a serial
>link. Using the 10 MHz, the second is broken in millisecond slots and
>then further in 125 microsecond slots. Each 125 us slot can carry a
>message signaling some event to occur in the accelerators. On the
>receiving side, we have home-made PCI, PMC and VME boards which can
>react to any of these messages by starting a counter, generating an
>output pulse on their front panel or an interrupt on the bus. This
>allows hardware and software synchronization all around the complex (the
>LHC is 27 km in circumference).
>
>Now, to come to your question: we rely heavily in the PPS containing
>exactly 10 million ticks of the 10 MHz. All our UTC time-tagging logic
>is based on that, so we definitely need a stable phase between the 10
>MHz and the PPS. But as Tom says, I thought you got that by design in
>every GPSDO. Maybe we have misunderstood the question?

I read his original message to say his customers were asking for coicidence 
of pulse edge, rather than a finite (not zero) phase relationship.



>Cheers,
>
>Javier
>
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